The present invention relates to an improved process for the co-production of 6-aminocapronitrile (ACN) and hexamethylenediamine (HMD) by treatment of adiponitrile (ADN) with hydrogen in the presence of a nickel-containing catalyst at temperatures not below room temperature and elevated hydrogen partial pressure in the presence or absence of a solvent.
The present invention further relates to a process wherein the treatment of ADN is carried out in suspension or in a fixed bed in a downflow or outflow process.
The hydrogenation of ADN to 6-aminocapronitrile in the presence of solvents, especially ammonia, and nickel-containing catalysts has been described in detail for example in U.S. Pat. No. 2,762,835, U.S. Pat. No. 2,208,598 and WO 92/21650.
The nickel-containing catalysts used in the hydrogenation of ADN lose activity in long runs and therefore have to be replaced with new catalysts once the activity has dropped below a certain value.
Nickel-containing catalysts are widely used in industry for steam reforming, for methanization and for the hydrogenation of functional groups such as CO double bonds, C--C multiple bonds or nitrile groups. In many of the aforementioned applications the catalyst is deactivated sooner or later through the formation of carbonaceous deposits on the active catalyst surface. The formation of carbonaceous deposits in steam reforming and the removal of these layers by reaction with oxygen, hydrogen, steam or carbon dioxide is described in Trimm, Catal. Rev.Sci. Eng., 16(2), 155-187 (1977). Measurable reaction rates are achieved with hydrogen only at temperatures above 550.degree. C.
The regeneration of catalysts coated with carbonaceous deposits is generally effected by burning off the organic coatings with nitrogen-air mixtures. However, this method can be used only with catalysts which remain stable on reaction with air supported catalysts with a stable structure of oxidic material, such as SiO.sub.2, Al.sub.2 O.sub.3, TiO.sub.2, can be successfully regenerated by this method. For instance, GB-A 2,284,163 describes the regeneration of a supported catalyst with Pt, Pd, Ru, Rh, etc. or nickel by treatment of [sic] a gas containing at least chlorine and oxygen.
Catalysts with very high metal contents become damaged on burning off the organic deposits with air, altering their mechanical properties (see EP-A 61,042).
EP-A 61,042 also discloses that nickel-containing catalysts having a maximum nickel content of 50% by weight for the hydrogenation of butynediol to butanediol can be regenerated by hydrogen treatment at temperatures between 200 and 500.degree. C., preferably at temperatures above 275.degree. C.
Similarly, U.S. Pat. No. 5,310,713 describes a regeneration with hydrogen for an alkylation catalyst which may contain nickel, but the regeneration with hydrogen is carried out in the presence of liquid alkane and of a chloride source.
It is known from Journal of Catalysis 143 (1993), 187-200, that a nickel catalyst (25% by weight of Ni on SiO.sub.2) which is used for the hydrogenation of acetonitrile in the gas phase can be regenerated by treatment with hydrogen at temperatures of above 200.degree. C.
The cited references do not reveal whether it is also possible to regenerate nickel-containing catalysts used in the hydrogenation of higher boiling dinitriles, especially adiponitrile. For bi-functional compounds such as dinitriles, in particular, can give rise, under reaction conditions, to the formation of oligomers which lead to regeneration problems.